Tokkai H8-285808 published by the Japanese Patent Office in 1996 discloses a device for diagnosing deterioration in an air/fuel ratio sensor which is provided in the exhaust passage of an internal combustion engine. In this prior art, a determination is made as to whether the air/fuel ratio sensor has deteriorated or not by performing air/fuel ratio feedback control of the engine and comparing change in a feedback correction coefficient and change in the air/fuel ratio detected by the air/fuel ratio sensor.